PROJECT SUMMARY/ABSTRACT The production of type-1 interferons (IFNs) by the host innate immune system presents the first barrier against viral infection. IFNs are innate immune factors that upregulate expression of hundreds of IFN- stimulated genes (ISGs), which results in induction of an ?antiviral state?. A small group of ISGs encode proteins that restrict HIV-1 and SIV replication and are referred to as ?restriction factors?. Restriction factors are less active against wild-type viruses replicating in their natural host but act as potent barriers against cross- species transmission. Macaque model systems are critical gatekeepers for testing HIV-1 prevention methods and for studies of HIV-1 transmission and pathogenesis. HIV-1 does not persistently infect macaques due to restriction by several macaque-specific restriction factors necessitating the use of chimeric SIV/HIV-1 viruses (SHIVs). Existing SHIV/macaque models typically employ SHIVs that encode HIV-1 sequences from viruses amplified in culture and further adapted in macaques (adapted SHIVs). Development of SHIVs encoding circulating HIV-1 variants derived directly from infected humans (circulating SHIVs) has been challenging as these SHIVs replicate poorly in macaque cells, if at all. While some host restrictions to HIV-1 replication in macaques have been defined, there is limited information on macaque-specific restriction factors that limit replication of circulating HIV-1 variants. Our preliminary results suggest that circulating SHIVs replicate poorly and are potently inhibited by macaque-specific IFN responses despite encoding the SIV antagonists of known restriction factors. In contrast, SHIVs encoding adapted HIV-1 sequences are resistant to IFN inhibition. Thus, this research proposal will characterize the host-viral interactions that selectively restrict replication of circulating SHIVs. During the mentored phase (K99): 1) the viral determinants of macaque-adapted SHIVs that confer resistance to IFN are expected to be defined, and 2) macaque-specific restriction factor(s) against circulating HIV-1 variants is expected to be identified. During the independence phase (R00): 1) characterization of the viral determinants of macaque-adapted SHIVs will be performed. The ability of the adaptive mutations to infect targets cells at the sites of mucosal transmission will be determined, 2) a novel example of cross-species host-viral interaction will be explored by characterizing the post-transcriptional regulation of HIV-1 envelope gene-expression in macaque lymphocytes, and 3) the mechanism of restriction of the macaque-specific restriction factor will be elucidated. Upon completion, this research proposal will successfully integrate the features of clinically relevant circulating HIV-1 variants with species-specific host innate immune system to help understand how macaque- specific IFN responses restrict circulating SHIVs.